Service Factors

To determine if a Silverthin™ Slewing Ring bearing is appropriate for an application,
a SERVICE FACTOR is applied. Refer to the table below for a guide to the service
factor to apply to your application. The load rating curves shown in this catalog are
approximate, and represent an application service factor of 1.00. To determine the
required bearing rating, multiply the applicable service factor by the applied loads
on the bearing, and compare the resultant loads to the load rating curves.

Class ofService

Typical Considerations

Application Examples

MinimumServiceFactor

LIGHT

Well defined loading

Tire mounted light duty construction

1.00

Loading well below capacity

Light duty Index table

1.00

Rotation slow, <10% of time and intermittent

Light duty industrial manipulator or robot

1.00

Light duty hand operated mechanism

1.00

Light duty medical devices

1.00

Light duty aerial platforms

1.00

Welding positioners

1.00

Rotating signs, displays

1.00

MEDIUM

Well defined loading

Track mounted light duty construction

1.10

Loading near or below capacity

Scrap yard construction

1.25

Rotation slow, <30% of time and intermittent

Medium duty industrial manipulator or robot

1.25

Conveyors

1.10

Rotary tables

1.25

Capstans and turnstiles

1.10

Wastewater treatment

1.10

HEAVY

Loading not well defined

Forestry handling equipment

1.50

Loading beyond machine capacity can occur

Heavy duty index tables and turntables

1.50

Shock loading can occur

Excavators

1.50

Rotation intermittent, up to 100% of time

SPECIAL

Loading not well defined

Alternative energy (wind, hydro, etc)

TBD

Continuous rotation

Offshore application

TBD

High speed rotation

Amusement rides

TBD

Heavy loads, shock, impact

Steel mill applications

TBD

High precision, positioning

Precision robotics

TBD

If you require any assistance in determining an applicable service factor, or would
like a more detailed load rating curve (recommended if your service factor adjusted
applied loads fall close to, or beyond, the load rating curves shown in this catalog),
please contact Silverthin™ Engineering for assistance. Please note that the
equipment designer is responsible for determining the correct service factor, often
validated by testing.

Typical Application

“Typical application” of Silverthin™ Slewing Ring
Bearings will exhibit the conditions listed below.
Special consideration must be given to bearing
selection and features whenever the application
conditions differ from those considered “typical”.
Those typical application conditions are:

For single row bearings, intermittent rotation (not continuous) should not
exceed a pitch-line velocity of 500 feet/minute.

Operating temperature between -40ºF to +140ºF.

Mounting surface geometry and installation procedures to assure roundness and
flatness of both races. An example approach would be to apply a centered thrust
load while tightening the bolts using the alternating star pattern method.

Load Capability

This is accomplished in most cases by
the unique four point contact raceway
geometry, which is similar in concept
to Silverthin™ X-Type Thin Section
bearings. This allows a single bearing
to accommodate all three loading scenarios noted
above, either individually or a combination thereof.

Speed

Accuracy

Silverthin™ Slewing Ring Bearings are not typically
provided with diameter tolerances. Some slewing ring
applications require a higher degree of accuracy. For
engineering and design support on special applications
please contact Silverthin Engineering.

Environment

Silverthin™ Slewing Ring Bearings are often used
indoors, and outdoors where exposure to moisture
and significant contamination is possible. Normal
temperature ranges -40°F to +140°F (-40°C to +60°C)
are standard. Slewing rings designed to operate in
harsher environments are available from Silverthin,
contact a Silverthin Engineer early in your design
process to identify the best bearing system solution
for extreme environments.

Mounting – Tension versus Compression

As mentioned earlier, it is best to mount the bearings
in “compression” as shown below. This ensures that
the load is carried by the balls, which is represented
in the load curve provided. Tension mounting has
significantly less capacity, as then the bolt strength
becomes the primary consideration for capacity.

Mounting

Mounting surfaces need to be machined accurately for
proper function of the bearing. Where standard bolt
patterns cannot be accommodated, contact Silverthin
Engineering for alternative options. Consideration
must be given to mounting in tension or compression.
In tension, BOLT strength becomes the limiting load
consideration, the load curve no longer applies, and
special considerations must be made. See additional
guidelines below.

Minimum Mounting Structure Guidelines

Generally, this rule of thumb will provide adequate
structural integrity.

Flatness & Mounting Surface Dish (Mounting Surface)

Flatness of the bearing mounting surface is critical to
optimal performance. Frequently mounting structures
are welded or worked in a way to induce stresses into
the structure. These stresses must be relieved, following
which the bearing mounting surface must be machined
flat. Flatness must be considered:

Circumferential Direction (δr): The amount of out-of-flatness
allowable in the circumferential direction for
four-point ball bearings is shown in the figure below.
This amount of out-of-flatness must not be exceeded
in a span less than 90°, and not more than once in a
span not more than 180°.

Allowable Dish or Perpendicularity Deviation in the
Radial direction (δp): For four-point contact ball
bearing designs, this amount of dish allowable can be
approximated using the formula:

δp ≈ 0.001 ∗ Dw ∗ P

Where:

P

=

radial dim of the mounting structure face (in)

Dw

=

rolling element diameter (in)

Note that if an application requires greater precision or
low rotational torque, it may be necessary to reduce
the values of δr and δp. For roller bearings, the amount
of flatness allowable is approximately 2/3 of that for an
equivalent sized four-point contact ball bearing.

Lubrication

Grease is the most common lubricant used in
slewing ring bearings and gear applications. Regular
lubrication through provided grease fittings or grease
holes is required for proper operation on standard
slewing rings. For special lubrication options, contact
Silverthin.™

Friction Moment (Rotation Torque)

The Friction Moment can be estimated for a slewing
ring bearing using the formula noted below. The
resulting values assume that the bearing is mounted
according to the guidelines outlined in this catalog.
This estimate only applies when load is applied to the
bearing, and does not reflect starting torque in an
unloaded condition. Also not considered are frictional
torque generated by the lubricant, seals and weight
of the components. This does however provide
a starting point, and with additional experience
adjustments can be made in the assembly to
accommodate for additional torque.

Where:

Mf = μ ∗ (4.4M + Fa Dpw + 2.2 Fr Dpw) / 2

Mf

=

Bearing starting torque under load (ft-lbs)

μ

=

Coefficient of friction (0.006 typically)

M

=

Moment load (ft-Ibs)

Fa

=

Axial load (Ibs)

Fr

=

Radial load (Ibs)

Dpw

=

Bearing pitch diameter (ft)

Bolts

It is always suggested that bolts be selected with
the advice and assistance of a fastening hardware
supplier. Bolt quality, pretensioning procedures, and
maintenance can vary widely.

The optimal bolting arrangement has a bolt circle in
both the inner and outer races with equally spaced
fasteners. This results in a more uniform mounting
arrangement, yielding the best performance between
the bearing and the fasteners. This is not always
possible due to mounting structure arrangements,
and holes may be shifted accordingly. In these
cases testing is recommended to determine actual
bolt loads, validate joint configuration and assembly
procedure.

As a starting point to determine the approximate load
on the heaviest loaded bolt, the following formula
can be used. Please note that Silverthin™ makes
no warranty, expressed or implied, regarding bolt
adequacy. It is strongly recommended that testing be
performed to determine the actual load, as this is the
only reliable way to be certain.

RB =

12 ∗ M ∗ r

±

Fa

BC ∗ n

n

Where:

RB

=

Total load on heaviest loaded bolt (Ibs)

M

=

Moment load (ft-Ibs)

r

=

Rigidity factor. Use 3 for bearings and support structures of average stiffness.

Fa

=

Axial load (lbs)

If Fa is in tension, the sign is +

If Fa is in compression, the sign is -
Refer to section “Mounting - Tension versus Compression”

Securing Bearing to the Mounting Surface

When installing the bearing, it is important to
ensure that the bearing is as round as possible.
This will optimize load distribution and promote the
smoothest operation. The following procedures are
recommended as an aid.

Use hardened round flat steel washers in accordance
with ASTM F436 under the head of the bolt, and also
the nut. Lockwashers, and locking compounds on the
thread, are not recommended.

Install the washers, nuts and bolts in the bearing
and supporting structure and hand tighten. Do not
distort the bearing in order to install bolts. Apply a
moderate centered thrust load to the bearing. Tighten
the bolts to the equipment designer’s specifications.
A common approach is to use a star pattern to
tighten the bolts, sequences as shown in the diagram
below. The pattern is usually done in 3 steps at
approximately 30%, 80% and 100% of the final bolt
torque or tension level specified by the equipment
designer.

Loss of proper tension can lead to premature bolt
failure, failure of the bearing and structure, damage
to components, and fatality or injury to anyone in
the vicinity. The bolts require frequent inspection for
proper tension, which is commonly accomplished by
measuring torque of the bolt.